A dual fuel engine is an engine that includes a first fuel source that is utilized as the sole fuel source during certain operating conditions and a second fuel source that is integrated with the first fuel source at other operating conditions. In certain applications, the first fuel source is a diesel fuel and the second fuel source is natural gas. The diesel fuel provides the initial, low load levels of operation and is used for ignition for the natural gas at higher load operations. The substitution of natural gas for diesel fuel improves high load performance and emissions reduction, particularly when the engine is employed at locations where natural gas is abundant or available at low cost.
When the engine is operating in dual fuel mode, natural gas is introduced into the intake system. The air-to-natural gas mixture from the intake is drawn into the cylinder, just as it would be in a spark-ignited engine, but typically with a leaner air-to-fuel ratio. Near the end of the compression stroke, diesel fuel is injected, just as it would be in a traditional diesel engine. The diesel fuel ignites, and the diesel combustion causes the natural gas to burn. The dual fuel engine combusts a mixture of air and fuel in the cylinders to produce drive torque. A dual fuel engine can operate either entirely on diesel fuel or on the substitution mixture of diesel and natural gas, but cannot operate on natural gas alone. However, the dual fuel engine typically delivers the same power density, torque curve and transient response as the base diesel engine does.
Dual fuel engines encounter difficulties during operation to provide the optimal balance of diesel fuel energy and natural gas energy. The amount and heating value of diesel fuel that is delivered to the combustion chambers of the cylinders is readily determined, and thus the energy provided by the diesel fuel can be readily determined. However, it is difficult to determine an accurate flow rate or quantity of natural gas that is delivered to the cylinders. In addition, the heating value of natural gas is not constant. As a result, current techniques for determining and providing a substitution rate of natural gas for diesel fuel in the dual fuelling mode often result in less than the optimal balance between diesel fuelling and natural gas fuelling being achieved. Thus, there remains a need for additional improvements in systems and methods for providing and controlling fuelling in dual fuel engines.